The influence of segment endpoint kinematics on segmental power calculations

Segment power can be estimated from the rate of change of segment energy or from segment endpoint dynamics. The inequality between these two calculations is termed the segment power imbalance (ΔP). Explanations for this imbalance have been largely anecdotal. We used 3D gait data for 20 young, healthy adults to show experimentally that ΔP is always zero when rigid body assumptions are strictly adhered to. We show mathematically and experimentally that ΔP is non-zero (range: <1–100 W) when segment endpoints move relative to the segment center of mass (COM). We also show that Lagrangian numerical differentiation of segment endpoint trajectories produce small velocity artifacts which result in observable ΔP (range: 0.3–5 W). The benefits of obtaining a small ΔP was found to be offset by large discrepancies (range: 8–54 W) in joint power across the joint gap. For the foot, large ΔP (range: 23–106 W) was observed when the distal velocity component included velocity of the center of pressure relative to the footʹs COM. Our results suggest that, unless interpreted carefully, segment power calculations can potentially lead to erroneous clinical judgement.